1. Field of the Invention
The disclosed invention relates to a semiconductor device using a semiconductor element and a driving method of the semiconductor device.
2. Description of the Related Art
Memory devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile memory device is a static random access memory (SRAM). Since an SRAM holds stored data with a circuit such as a flip flop, the number of elements per memory cell is increased (for example, six transistors per memory cell); therefore, cost per storage capacity is increased.
Another example of a volatile memory device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory cell is selected and charge is accumulated in a capacitor.
In a DRAM, a transistor included in a memory cell has off-state current (leakage current between a source and a drain when the transistor is off) or the like and charge flows from or into a capacitor even when the transistor is not selected. The invention disclosed in Patent Document 1 proposes that a structure of a transistor in which a channel is formed in a semiconductor substrate including silicon is devised and the off-state current of the transistor is reduced. However, it is difficult to sufficiently reduce the off-state current of the transistor in which the channel is formed in the semiconductor substrate including silicon. Therefore, operation of writing data in a memory element (refresh operation) needs to be performed at predetermined intervals (e.g., several tens of times per second) in a conventional DRAM even in the case where stored data is not rewritten; accordingly, it is difficult to sufficiently reduce power consumption of the DRAM.